The coral reef that lies off Key Largo forms part of the Florida Reef tract, the largest living coral reef system in North American waters and the third-largest barrier reef in the world, after Australia's Great Barrier Reef and the Belize Barrier Reef. Several kilometres from the coasts, it skirts almost 322km of coastline, from Fowey Rock (south of Miami) to Dry Tortugas, descending to depths of up to 24m. Thankfully, this fragile ecosystem is protected by the vast Florida Keys National Marine Sanctuary.